1. Field of the Invention
The present invention relates to a magnetoresistive effect device, a magnetic random access memory (MRAM), and a magnetoresistive effect device manufacturing method, and particularly to a structure of a magnetic memory cell configured to utilize an device in which storage of information of “1” or “0” is performed by the tunneling magnetoresistive effect.
2. Description of the Related Art
Recently many memories in which the information is stored by a new principle are proposed. One of the newly proposed memories is an MRAM in which magnetic memory cells are arranged in a matrix shape. The magnetic memory cell is formed by a ferromagnetic tunnel junction (MTJ) device in which the storage of information of “1” or “0” is performed by utilizing the tunneling magnetoresistive effect (see Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCCC 2000 Technical Digest pp. 128 to pp. 129). An MRAM has the features of a nonvolatile property and high speed.
The MTJ device includes a barrier layer and two magnetic layers. A magnetization direction of one of the magnetic layers is typically fixed by an antiferromagnetic layer. In the other magnetic layer, the magnetization direction can be changed by an external magnetic field. The magnetic layer whose magnetization direction is variable is referred to as, e.g., recording layer, and the magnetic layer whose magnetization direction is fixed is referred to as, e.g., fixed layer.
Magnetization switching of the magnetic layer is shifted (changed) according to a leakage magnetic field from other magnetic layers, surface roughness, and the like. That is, a hysteresis loop of a magnetization curve of a magnetic body is shifted from an original position. The hysteresis loop is expressed by the magnetic filed (H) and the magnetization (M). Conventionally, the shift caused by the leakage magnetic field from the fixed layer and the shift caused by roughness of a laminated structure exist simultaneously in the magnetization switching shift of the recording layer, so that a switching magnetic field fluctuation is remarkably large among the devices it is preferable that magnitude of the magnetization switching shift of the recording layer is 1 Oe or less, and it is more preferable that the magnetization switching shift is substantially eliminated. Since it is naturally difficult to eliminate the roughness, the recording-layer magnetization switching shift caused by the leakage magnetic field from the fixed layer and the magnetization switching shift caused by the roughness cannot substantially be eliminated at the same time. Therefore, conventionally the thickness of the fixed layer is adjusted to substantially eliminate the magnetization switching shift such that the shift caused by the roughness and the shift caused by the leakage magnetic field cancel each other. However, in this adjustment method, the magnetization switching and the shift of the recording layer are fluctuated because a thickness may vary and a processing may be fluctuated.
Jpn. Pat. Appln. KOKAI Publication No. 2004-128237 discloses a techniques in which the saturation magnetization of a first ferromagnetic layer 23c is set smaller than the saturation magnetization of a second ferromagnetic layer 23a based on “Effect of finite magnetic film thickness on Neel coupling in spin valves”, J. C. S. Kools et al., J. Appl. Phys. 85 (1999) p. 4466–68. That is, in Jpn. Pat. Appln. KOKAI Publication No. 2004-128237, a difference in magnetic moment per unit area between the first ferromagnetic layer and the second ferromagnetic layer is increased to substantially eliminate only the recording-layer magnetization switching shift caused by the roughness. However, since the difference in magnetic moment per unit area between the first ferromagnetic layer and the second ferromagnetic layer is large, the shift caused by the leakage magnetic field does not become substantially zero. Further, in the case where the thickness difference is large, a large distance may be provided between the end of the fixed layer and the recording layer to relatively decrease the shift caused by the leakage magnetic field. However, this technique is not suitable for miniaturization and may not be realistic.